DETERMINING THE LONGITUDE. 241 



the equator, by merely giving the instrument a sweep slowly along the meri- 

 dian, noting the times of transit and the altitude, or zenith distance of each 

 star on the circle. From the altitudes, he computes the approximate declina- 

 tion of the star, and knowing also, from the time of its transit, the approximate 

 right ascension of the star, he will, on reference to the catalogues, be able to 

 discover its name and character. 



It may, however, be laid down as a rule, that when an observer can select 

 from ten to fifteen or twenty good stars, that is, stars whose right ascensions 

 have been recently determined,* some before and some after the moon's 

 transit, and within two or three hours, a desirable degree of accuracy will be 

 attained j and when a good clock or chronometer is used, the interval may be 

 extended to five or even six hours ; but when a watch is used, whose rate is 

 not uniform, the interval ought not to exceed one hour, or at most two hours ; 

 and it should ever be borne in mind, that one good observation of a star whose 

 ARn. is well determined, is far preferable to a hundred observations of stars, 

 whose ARns. are imperfectly ascertained. 



It has elsewhere been observed, that when a star is on the meridian, the 

 right ascension of that star is the right ascension of the meridian ; it follows, 

 that when the moon's limb is on the meridian, the right ascension of that limb 

 is the right ascension of the meridian. But the right ascension of the meri- 

 dian may be deduced from the interval in sidereal time that elapses between 

 the transit of a star, and any given instant of time j consequently, if at any 

 known interval, before or after the transit of a star, the moon's limb be on 

 the meridian, the right ascension of the meridian deduced from that interval, 



* Pond's Catalogue of 400 stars is, probably, the most accurate that has been published, and 

 I have used it in preference to any other. 



2 N 



